| In the arid and semiarid regions of the mid-latitude Asia, the natural landscape is dominated by desert (desert and Gobi), and climate is mainly controlled by the westerly circulation. The climate combination of the arid and semiarid regions of the mid-latitude Asia is warm-dry and cold-wet during the last interglacial stage and the last millennium. However, under the background of global warming, the temperature in the arid and semiarid regions of the mid-latitude Asia in the past century has undergone rapid rise, which leads to the changes of atmospheric circulation occur, and impacted on the arid and semiarid regions of the mid-latitude Asia, and showed the warm-wet trend. On the whole, anti-phase or out-of-phase relationships in drought changes between the the inland of the arid Asia and monsoonal Asia were existed on centennial to millennial timescale in the Holocene, and it is thus proposed that there is a ’westerlies-dominated climate regime’ in mid-latitude Asia during the last interglacial stage and the last millennium. Although paleoclimate studies of the arid and semiarid regions of the mid-latitude Asia has made a lot of progress in recent years, the spatial and temporal resolution of the climate reconstruction is still low, and lack of the combinations of temperature and drought reconstruction, and make bad understand about the spatial patterns of the arid and semiarid regions of the mid-latitude Asia and its driving mechanism. Therefore, the temperature and drought reconstruction and the impact of climate change is the scientific issues, and also link with the sustainable development of the arid and semiarid regions of the mid-latitude Asia under the context of global warming. In this paper, based on the tree-ring width and density data from the arid and semiarid regions of the mid-latitude Asia, we reconstructed temperature and drought series in the arid and semiarid regions of the mid-latitude Asia (China, Tajikistan and Kyrgyzstan), and analyzed climate combinations and the spatial patterns of drought in the arid and semiarid regions of the mid-latitude Asia, and combined with modern climate data in different sites to study climate mechanism.First, based on tree-ring width and density data, we developed the reconstructions of regional drought and temperature respectively, and analyzed climate combinations of the different areas:(1) The Loess Plateau of Gansu have shown the warm-dry and wet-cold combinations on interannual and decadal scales during warm season, and affected by the East Asian summer monsoon activity during the last 380 years. During the strong monsoon period, the enhanced water vapor transport and increased precipitation can cause the temperature to drop, and vice versa. (2) The Hexi Corridor has the wet-warm and cold-dry combinations during warm season. However, due to the activities of climate system, such as Asian summer monsoon, sea surface temperature, the combination is not stable. (3) Northern Xinjiang also have shown a warm-dry and wet-cold combination on the interannual and decadal scales during the 221 years, however, warm and wet trend was found since the 1980s. Overall, the climate combination of warm season in the arid and semiarid regions of the mid-latitude Asia on the decadal scale is warm-dry and cold-wet.Second, based on the drought reconstruction, we analyzed the spatial patterns of drought in mid-latitude Asia:(1) The comparison showed that the weak positive correlations were found among the drought series from the monsoonal areas of China, and exhibited strong consistency on the decadal scale. Therefore, the changes in the eastern and western part of the monsoonal region are dominated by the positive phase changes. (2) The drought sequence of the Loess Plateau of Gansu has good correlations with the drought sequence of the Hexi Corridor. The drought reconstructions of Gansu have a good response to extreme events of Northwest China. At the same time, there are also some regional differences between the drought reconstructions of Gansu, which may by affected the interaction of westerlies and Asian summer monsoon. (3) The drought reconstructions of northern Xinjiang show the strong common signals, and there are also some regional differences in different parts of northern Xinjiang. Among them, the most obvious difference is found between the eastern end of the Tien Shan and central-west Tien Shan. The central-west Tien Shan and the Altay Mountains appear wet trend during the recent years, while the eastern end of the Tien Shan appeared to dry trend, and strongly associated with the drought series from the monsoonal areas of China. Thus, the eastern end of the Tien Shan is likely to be an important geographical boundary in the arid and semiarid regions of the mid-latitude Asia. (4) The comparison between the Alai Mountains of Kyrgyzstan and Tajikistan and Tien Shan exhibited strong consistency on the decadal scale, and showed similar warming and wetting trend in the 20th century. The opposite trend in moisture variation between the Alai Mountains and central Mongolia was found. Of particular interest is the wet period of central Mongolia during the 1211-1225, which coincided with the rise of Chinggis Khan. In contrast, the drought reconstruction of the Alai Mountains exhibits a downward trend during 1211-1235. The adverse climate conditions reduced agricultural crop yields, diminished the political and military strength of the Khitan kingdom and Khwarezmia and favored the rapid expansion of the Mongols under Chinggis Khan.Due to geographical differences, the different driving mechanisms of climate were found in different regions of the arid and semiarid regions of Asia:(1) The reconstructed drought of west Tien Shan is significantly correlated with sea surface temperature in the North Atlantic and Indo-West Pacific Oceans. The linkages to the North Atlantic and Indo-West Pacific Oceans suggest the connection of moisture variations of Central Asia to the westerly circulation and tropical ocean-atmosphere systems. (2) Drought variations of the northwestern Altay region are positively correlated with sea surface temperatures in tropical oceans, suggest a possible linkage of the recipitation variation of the northwestern Altay region to the El Nino-Southern Oscillation (ENSO). The 500-hPa vector wind composite anomaly of the wettest years exhibit westerly and northerly flow over our study area. Temperature and precipitation composites suggest that extremely high reconstructed precipitation years are characterized by wet and slightly colder than average conditions. This is consistent with high-level transport of cold air from the mid-high latitude oceans, and vice versa. (3) The significant positive correlations between the reconstructed drought series of the Alai Mountains and sea surface temperatures are found in the tropical Indian Ocean, and suggesting that tropical Indian Ocean is the important water vapor sources for the Alai Mountains area. The wetting trend of the Alai Mountains area in the 20th century may be linked with the increase of sea surface temperatures. The 500-hPa vector wind composite anomalies of the wet and dry years during the period 1948-2013 support the connection with the SASM intensity. During the wet years, the study area was controlled by the westerly jet, and the enhanced southwesterly flow (SASM) brings abundant water vapour across the Arabian Peninsula and travels further northward, causing increased precipitation over central Asia, and vice versa. In order to further reveal the relationships between the drought variations of the western part of Central Asia and SASM at longer time scales, we analysed the correlations between our PDSI reconstruction and the SASM index for the period 1076-2000. The correlation between our PDSI reconstruction and the SASM index are 0.16 (P<0.001, n=925), and increase to 0.27 (P<0.001, n=925) after 20-years smoothing. Our preliminary analysis reveals that the SASM have important impacts on the drought variation of central Asia, however, the mechanism at various timescales awaits further explore.The last 30 years, a significant warming and wetting trend was found in Northern Xinjiang, and it may be the occasion of North Atlantic Oscillation, sea surface temperature, westerly activities and so on. The study found that temperature reconstruction of northern Xinjiang has significantly associated with central-west Gansu. At the same time, detailed analysis suggests a cooling of several years following primarily tropical events with a volcanic eruption index (VEI). The average temperature drop 0.4-0.6℃. Meanwhile, in the role of westerly circulation summer, volcanic eruptions from the westerly region have stronger influence than the tropical volcanic eruptions, especially Laki volcanic eruption in Iceland in 1783, and 1783 is the coldest year during the last three centuries.Based on the climate data and tree-ring series, the analysis results show that the spatial distribution of the monsoonal area of east China is of a meridional tripole pattern. Composite maps of the wettest and driest years in the recent past also reveal patterns that are similar to those indicative of ENSO events, particularly with regards to ocean and atmospheric conditions in eastern Pacific, revealing ENSO events as a causal mechanism for wet and dry conditions in southern China. Of particular interest are the shifts from the dry phases to the wet phases following primarily El Nino events. After warm phases of ENSO, the strength of East Asian summer monsoon (EASM) tends to be enhanced via the Walker circulation, and precipitation in southeast China increases. As discussed above, drought variations in southeastern China are probably related to ENSO and various parts of the remote ocean circulation. |
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